1. Field of the Invention
The present invention relates to an amplifier and, more particularly, to an amplifier constituted by transistors. The present invention is suitably used as a pre-amplifier for amplifying, e.g., a sensor signal and the like.
2. Related Background Art
FIG. 3 shows an equivalent circuit diagram of a two-dimensional sensor using a conventional pre-amplifier. Referring to FIG. 3, the sensor comprises photoelectric conversion cells each constituted by a bipolar transistor 1 constituting a sensor for accumulating carriers generated upon incidence of light energy at its base, a capacitor 2 for controlling the base potential of the bipolar transistor 1, and a p-MOS transistor 3 for resetting the base potential of the bipolar transistor 1. The sensor further comprises drive lines 4 for driving each cell, vertical output lines 5 connected to the emitters of the bipolar transistors 1, buffer MOS transistors 6 each of which is selected by a vertical shift register and supplies pulses to the drive line 4 to be driven, a pulse input terminal 7 for applying drive pulses, capacitors 8 for accumulating output signal voltages from the cells, MOS transistors 9 each for switching between the vertical output line 5 and the capacitor 8, a horizontal output line 10, MOS transistors 11 each of which is selected by an output from a horizontal shift register, and transfers a signal voltage from the corresponding capacitor 8 onto the horizontal output line 10, a MOS transistor 12 for resetting the horizontal output line 10, MOS transistors 13 for resetting the sensor cells, the vertical output lines 5, and the capacitors 8, a power supply terminal 14 for applying a reset voltage as a reference voltage of this sensor system, a terminal 15 for applying pulses to the gates of the MOS transistors 9, a terminal 16 for applying pulses to the gates of the MOS transistors 13, an amplifier MOS transistor 17 whose gate is connected to the horizontal output line 10, a load MOS transistor 18 whose well and source are connected to the drain of the amplifier MOS transistor 17, and whose gate and drain are connected to the power supply voltage, a terminal 19 for applying pulses to the gate of the MOS transistor 12, and an output terminal 20 of a pre-amplifier defined by the amplifier MOS transistor 17 and the load MOS transistor 18.
FIG. 3 exemplifies the sensor having 2.times.2 cells for the sake of simplicity. The operation of this conventional two-dimensional sensor is described in Japanese Patent Application Laid-Open No. 63-186466.
The characteristics of the pre-amplifier defined by the MOS transistors 17 and 18 will be described below with reference to FIG. 4.
If the transconductances of the MOS transistors 17 and 18 are respectively represented by g.sub.m1 and g.sub.m2, the ratio .DELTA.V.sub.out /.DELTA.V.sub.in of a change .DELTA.V.sub.out in output voltage to a change .DELTA.V.sub.in in input voltage, i.e., the gain, is given by: ##EQU1## Both the MOS transistors 17 and 18 have constant transconductances g.sub.m1 and g.sub.m2 in a saturation region, and the amplifier output becomes linear with respect to the input. If the linear input range is assumed to be a range between V.sub.1 and V.sub.2, these voltages V.sub.1 and V.sub.2 are respectively theoretically given by V.sub.1 =V.sub.th and V.sub.2 =V.sub.DD /(1+g)+g/(1+g).multidot.V.sub.th (where V.sub.th is the threshold value of the MOS transistors 17 and 18, V.sub.DD is the power supply voltage, and -g is the gain of the amplifier). The input voltage to this amplifier ranges from V.sub.VC to (V.sub.VC +V.sub.SAT) (where V.sub.VC is the reference reset potential of the sensor and V.sub.SAT is the saturation voltage of the sensor). When the amplifier is set to satisfy V.sub.1 &lt;V.sub.VC and (V.sub.VC +V.sub.SAT)&lt;V.sub.2, as shown in FIG. 4, the sensor output can be amplified with high linearity.
However, in the above-mentioned prior art, since the threshold voltage V.sub.th of the MOS transistor for defining the linear input range of the pre-amplifier varies due to a variation in an element formation process or a change in operation temperature, the linear input range (V.sub.1 to V.sub.2) of the amplifier may deviate from the amplifier input range (V.sub.VC to V.sub.VC +V.sub.SAT). When V.sub.VC is set to be relatively high to satisfy V.sub.th (=V.sub.1)&lt;V.sub.VC even if the threshold voltage V.sub.th varies, and the linear input range is widened to satisfy (V.sub.VC +V.sub.SAT)&lt;V.sub.2, the gain g of the amplifier must be decreased.